DF9: The Third Galaxy Without Dark Matter ⚡ экспресс
A study of galaxy DF9 from the chain of objects in the NGC 1052 field confirms that it lacks dark matter, like previously studied DF2 and DF4. Using the KCWI instrument on the Keck telescope, the stellar velocity dispersion was measured: 6.5^{+3.9}_{-4.3} km/s. This is consistent with an expected dispersion of 8.3^{+0.9}_{-1.4} km/s for a stellar mass of 1.4×10^8 M⊙ without dark matter. If DF9 had a typical dark matter halo of 1.4×10^10 M⊙, the dispersion would be 24±3 km/s, which is excluded at the >3σ level. The result does not depend on the assumed distance to the chain (details in Appendix B). This is the third confirmation of a unique formation channel—the 'dwarf bullet' scenario, which predicts the absence of dark matter in all galaxies of this group.
In the constellation Cetus, 65 million light-years from Earth, three dwarf galaxies nestle: DF2, DF4, and DF9. All have been declared “galaxies without dark matter” — the mysterious invisible substance that usually holds stars together. Its existence was suspected by Fritz Zwicky and Vera Rubin back in the last century, observing the overly fast motion of distant objects.
Astronomers measured how fast stars move in DF9. It turned out they amble at a pedestrian’s pace — about 6.5 km/s. For comparison: our Sun races around the Milky Way’s center 30 times faster. Normally, in galaxies stars dart about, propelled by invisible mass. Here, their unhurried step suggests that all the weight comes only from the stars themselves. Dark matter is not to blame.
The secret of these galaxies’ birth is as simple as a kitchen sieve. Long ago, two dwarf systems collided head-on. Their gas — ordinary matter — got stuck and clumped, like pasta in a colander. Meanwhile dark matter, interacting with nothing, seeped on through, like water. Eventually, stars were born from the sticky gas, forming a chain of galaxies left without a dark component. This happens extremely rarely, but it exactly explains the existence of DF2, DF4, and DF9.
🎯 If you imagine galaxy DF9 as a city street, its stars are pedestrians strolling at 6.5 km/s. Meanwhile, the Milky Way is a racetrack, where stars zoom like Formula 1 cars under the guidance of an invisible dispatcher.